MONDAY, Feb. 24, 2014 (HealthDay News) -- About one in 10 heart surgery patients has persistent pain for up to two years after the operation, a new study reveals.

The study included more than 1,200 patients aged 18 and older who had heart procedures -- such as bypass or valve replacement -- at four cardiac surgery centers in Canada.

Postoperative pain was felt by 40 percent of patients after three months, 22 percent of patients after six months, 17 percent after one year, and 10 percent of patients after two years, according to the study in the current issue of the CMAJ (Canadian Medical Association Journal).

"These prevalence rates are not negligible given that more than 400,000 patients undergo [heart bypass] annually in the U.S.," wrote Dr. Manon Choiniere, a researcher with the University of Montreal Hospital Research Centre, and colleagues.

The researchers also found that those with increased risk of pain two years after heart surgery included: younger patients; those who had chronic pain before the operation; those who were more anxious before the procedure; and those who had higher levels of pain during the first week after surgery.

"It is noteworthy that acute pain levels (observed in the first few days after surgery) were quite high in this study," the authors wrote. "It is troublesome that similar observations have been made over and over again in the past three decades and that postoperative pain continues to be undertreated despite numerous awareness campaigns, pain care guidelines and educational efforts."

The researchers said anxiety before surgery and pain severity in the first few days after surgery are modifiable risk factors for long-term pain after heart surgery. And patients need to be informed about the risk of long-term chronic pain, which can reduce quality of life, the study authors pointed out in a journal news release.

Further research is needed to find ways to prevent or minimize long-term pain after heart surgery, the study authors concluded.